A glass-ceramic is a material having at least one crystalline phase thermally developed in a uniform pattern throughout at least a portion of a glass precursor. Glass-ceramics have been known for over 30 years since being described in U.S. Pat. No. 2,920,971 (Stookey). They find application in diverse areas, an area of particular interest being the fabrication of articles used in the preparation and serving of food. Such articles include cookware, bakeware, tableware and flat cooktops.
In general, production of a glass-ceramic material involves three major steps: melting a mixture of raw materials, usually containing a nucleating agent, to produce a glass; forming an article from the glass and cooling the glass below its transformation range; crystallizing ("ceramming") the glass article by an appropriate thermal treatment. The thermal treatment usually involves a nucleating step at a temperature slightly above the transformation range, followed by heating to a somewhat higher temperature to cause crystal growth on the nuclei.
Crystallization of glasses in the Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 composition field generally provides highly crystallized glass-ceramics. The primary crystal phase, depending on glass composition and heat treatment, may be a transparent beta-quartz solid solution, or an opaque beta-spodumene solid solution.
Beta-quartz is the hexagonal trapezohedral modification of SiO.sub.2. It exhibits a slightly negative coefficient of thermal expansion (CTE). This makes it of particular interest where thermal cycling occurs, as in cookware. The basis of the beta-quartz solid solution is believed to be the substitution of Al.sup.+3 ions for some of the Si.sup.+4 ions in the beta-quartz structure. The attendant charge deficiency is made up by the introduction of a small ion, such as Li.sup.+, Mg.sup.+2 , or Zn.sup.+2, into the beta-quartz structure.
Beta-quartz solid solution glass-ceramics customarily contain TiO.sub.2 as a nucleating agent. Optionally, the TiO.sub.2 may be partially, or wholly, substituted for by ZrO.sub.2. The appearance of such glass-ceramics can be varied by varying composition and/or heat treatment. Thus, transparent, translucent, or opaque glass-ceramics, which may be water-white, translucent, opaque white, or variously colored, are all possibilities as described in the prior art.
The widest use of Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 glass-ceramic materials has been in the field of culinary ware. For over three decades, Corning Glass Works, now Corning Incorporated, has marketed opaque white cooking utensils under the trademark CORNING WARE. More recently, cooking utensils, formed from a transparent glass-ceramic exhibiting a light brown tint, were introduced commercially by Corning France, S.A. under the trademark VISION. In general, this transparent glass-ceramic is crystallized at lower temperatures to develop small, beta-quartz solid solution crystals. Such glass-ceramics and their production are described, for example, in U.S. Pat. Nos. 4,018,612 and 4,526,872.
It has been observed that transparent, beta-quartz glass-ceramics nucleated with TiO.sub.2 tend to exhibit a light brown tint. This is ascribed to the presence of both TiO.sub.2 and Fe.sub.2 O.sub.3 in the parent glass composition. Efforts have, therefore, been made to either develop a decolorizer or to mask the tint. In an opaque, white glass-ceramic, the brown tint is effectively masked. Masking, however, becomes more difficult in transparent glass-ceramics.
The present invention arises from a desire to achieve a decorative color in a transparent, beta-quartz solid solution glass-ceramic. This goes beyond simply neutralizing the inherent brown tint. In addition to achieving the desired coloration, production demands that it be done while maintaining a certain degree of infra-red transmission in the molten glass. This is necessary to adequately retain heat in the glass during the forming process. Heretofore, this has been accomplished by maintaining the Fe.sub.2 O.sub.3 level in the composition at a small, but reasonably critical, amount.
It is apparent then that any effort at achieving a controlled color in a transparent, beta-quartz solid solution glass-ceramic must take into consideration the inherent color effects of TiO.sub.2 and Fe.sub.2 O.sub.3. It is known, of course, that color may be imparted to a glass by incorporating one or more colorants, usually transition metal oxides, in the precursor glass. However, the color in a parent glass may change markedly during the ceramming step. Therefore, the ultimate glass-ceramic color is often not predictable from the initial glass color.
U.S. Pat. No. 5,070,045 (Comte et al.) discloses transparent glass-ceramic plates that use 0.1-1.0% of a colorant selected from CoO, NiO, Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, MnO.sub.2, and V.sub.2 O.sub.5. The patent is primarily concerned with V.sub.2 O.sub.5 which is taught to contribute to minimal distortion while giving a black aspect in reflection and a reddish brown tint in transmission. The predominant crystal phase in the glass-ceramics is .beta.-quartz solid solution. Their compositions consist essentially, in weight percent, as calculated on the oxide basis, of:
______________________________________ SiO.sub.2 65-70 MgO + BaO + SrO 1.1-2.3 Al.sub.2 O.sub.3 18-19.8 ZrO.sub.2 1.0-2.5 Li.sub.2 O 2.5-3.8 As.sub.2 O.sub.3 0-1.5 MgO 0.55-1.5 Sb.sub.2 O.sub.3 0-1.5 ZnO 1.2-2.8 As.sub.2 O.sub.3 + Sb.sub.2 O.sub.3 0.5-1.5 TiO.sub.2 1.8-3.2 Na.sub.2 O 0-&lt;1.0 BaO 0-1.4 K.sub.2 O 0-&lt;1.0 SrO 0-1.4 Na.sub.2 O + K.sub.2 O 0-&lt;1.0 BaO + SrO 0.4-1.4 ##STR1## &gt;1.8 ______________________________________ U.S. Pat. No. 5,179,045 (Aitken et al.) describes production of a burgundy color in a glass-ceramic having as its primary crystal phase a beta-quartz solid solution. The glass-ceramic contains up to 6% TiO.sub.2 as a nucleating agent, and has a color package composed of 50-150 ppm Co.sub.3 O.sub.4, 50-250 ppm NiO and 400-1000 ppm Fe.sub.2 O.sub.3 to provide the desired burgundy color.
The color package described in this pending application has been the basis of commercial cookware available from Corning Incorporated that has a burgundy color. It would, of course, be desirable to obtain the different colors, as well as the opaque product, with a single base glass composition for the precursor glass. That would facilitate changing from one product to another with a single melting unit. Even more desirable would be the ability to melt one precursor base glass and then provide subsequent treatments to achieve the different colors. It is a primary purpose of this invention to meet these desirable ends.